feat: Complete Issue #38 TDD8 Cycle 1 - Content command family implementation

Implemented comprehensive content command family for MarkdownMatters CLI following TDD8 methodology and MarkdownMatters specification.

## TDD8 Cycle 1 - Content Commands

### Core Implementation
- Content parser for extracting main content without matter zones
- Content statistics calculator (words, lines, paragraphs, characters)
- CLI commands: `content-get` and `content-stats`
- Full integration with existing markitect CLI

### MarkdownMatters Compliance
- Correctly removes YAML/TOML/JSON frontmatter
- Correctly removes tailmatter blocks (`yaml tailmatter`, `json tailmatter`)
- Preserves contentmatter (MultiMarkdown key-value pairs within content)
- Follows three-zone specification from wiki/MarkdownMatters.md

### Module Structure
```
markitect/content/
├── __init__.py          # Module exports
├── parser.py           # ContentParser with matter zone removal
├── stats.py            # ContentStats data class
└── commands.py         # CLI commands implementation
```

### CLI Commands Added
- `markitect content-get --file [path]` - Extract pure content
- `markitect content-stats --file [path]` - Calculate content statistics

### Test Coverage
- 16 comprehensive tests covering all scenarios
- Test fixtures for different document types
- CLI integration tests with Click testing
- Edge case handling (file not found, empty content, etc.)

### Validation Results
- All tests pass (16/16)
- Manual CLI testing confirmed
- Proper matter zone separation validated
- Statistics calculation accuracy verified

## Technical Architecture

### ContentParser Class
- `extract_content()` - Remove frontmatter and tailmatter
- `calculate_stats()` - Generate comprehensive statistics
- `_remove_frontmatter()` - YAML frontmatter removal
- `_remove_tailmatter()` - Tailmatter block removal

### ContentStats Data Class
- word_count, line_count, paragraph_count, character_count
- JSON serialization support via `to_dict()`

## GAMEPLAN Progress
- ✅ TDD8 Cycle 1: Content Commands (COMPLETE)
- 🔄 Next: Cycle 2 - Frontmatter Commands
- Remaining: Contentmatter, Tailmatter command families

This implements the foundation for Issue #38 with 6 remaining cycles planned for complete MarkdownMatters CLI functionality.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

markitect/cli.py

@@ -3388,5 +3388,13 @@ def config_stats(config, format):
         sys.exit(1)
 
 
+# Content Commands (Issue #38)
+from .content.commands import content_get, content_stats
+
+# Register content commands
+cli.add_command(content_get)
+cli.add_command(content_stats)
+
+
 if __name__ == '__main__':
     main()